Evaluating the Accuracy of Inverter Based String IV Measurements

Inverter measurements of photovoltaic (PV) string current-voltage (IV) characteristic curves can support performance monitoring of PV plants without the need for additional equipment by using the intrinsic hardware capabilities of the inverter. This paper investigates the accuracy of such inverter based IV measurements of PV strings under various conditions, including shade, compared to a commercial PV array IV tracer. We find very good agreementfor high, stable irradiance conditions with a mean relative P_mp error of 0.6% at a maximum of 4%. At low irradiance, the error increased to 2.7% on average at a maximum of 6.7%. Changing irradiance led to higher uncertainty around the irradiance conditions at each measurement and therefore to higher errors with Isc, Imp, and Pmp, while Voc is usually well agreed. In limited shade conditions, the agreement is good, whereas the inverter started misinterpreting the shape of the IV curve when shade got more severe. At a G_POA of 220 W/m² or less, the inverter failed to take measurementsor missed inflection points caused by the non-uniform shade. This work demonstrates that the inverter can produce IV curves comparable to a high-accuracy IV tracer under non-shaded and stable irradiance conditions.